1. Field of the Invention
Embodiments of the present invention are directed to a probe head assembly used in semiconductor testing apparatus, and more particularly to a probe contactor substrate having probe contactors electroplated on a first side and machined mounting pads electroplated on a second side.
2. Description of Related Art
A modern probe card assembly used to test wafers of semiconductor chips generally consists of a Printed Circuit Board (PCB) (also referred to as a printed wiring board or probe card wiring board), a probe contactor substrate having probes for contacting the wafer (sometimes referred as a probe head), and an interposer connecting the PCB to the probe contactor substrate.
Probes (also referred to as probe contactors) are generally compliant mechanisms including at least one spring which have some limited range of compliance in a vertical direction (the “z” direction). Some probes have minimal or no compliance. When in use, a wafer under test is urged upward to contact the tips of the probes. In practice, there is some manufacturing process-related z error (non-planarity of the probe tips) caused by film stresses, etch control, assembly control, etc. as well as systemic z errors caused by a warping or curving in the surface of the probe contactor substrate. If the probe contactor substrate is curved or warped, so will be the imaginary surface that goes through the tips (assuming that the probes are of uniform height). Thus some probe tips will contact the wafer first (called the first touch z height) and some probe tips will contact the wafer last (last touch z height). Because probes generally have a limited range of compliance (as small as 50 μm or less for many compliant microfabricated technologies and effectively 0 for non-compliant technologies), it is desirable to minimize both the process-related and systemic errors in tip z height. Some errors are most directly related to the fabrication of the probes on the probe contactor substrate rather than the probe card assembly design. However, some errors are usually directly related to the probe card assembly and the way the probe contactor substrate (or substrates) are mounted to the rest of the probe card assembly. The minimization of these latter errors is the subject of the present invention.
In older probe card applications, a prober has a surface which has been planarized to that of the chuck that carries the wafer under test. The probe card PCB is generally mounted to this planarized surface of the prober. Thus, such probe card assemblies require well controlled parallelism between the plane of the probe tips (the best-fit plane that minimizes the overall root-mean-square z error between the probe tips and the plane) and the plane of the PCB (the PCB can be thought of as the “reference plane”). If the probe tips are co-planar with the PCB, then they are also co-planar with the chuck, and thus with the wafer under test. Such a design will lead to a more uniform contact of the probes to the wafer under test (less of a distance between first touch z distance and last touch z distance). In newer probe cards, the probe tips are referenced to mounting points on the probe card which are typically kinematic mounts of some type (used here to describe a mount that provides accurate and repeatable mechanical docking of the probe card into the test equipment and provides constraint in at least the two degrees of freedom necessary to achieve parallelism to the plane of the wafer chuck). In either case, it is necessary to align the tips of the probe contactors such that they are parallel to a reference plane which is itself parallel to the plane of the wafer chuck. Furthermore, it is desirable to mount the probe head to the probe card assembly in a fixed manner without the need for shimming or dynamic adjustment of the planarity of the probe card substrate once it has been mounted.
A common problem with mounting the probe head to the probe card assembly, is that the probe contactor substrate 100 to which the probe contactors 110 are attached is generally non-uniform and has thickness variations across its surface, as shown in FIG. 1. (While not shown in FIG. 1, the front side of the probe contactor substrate may also have variations in the in the thickness as well, however, the method of forming the probe contactors on the substrate accounts for this and allows the tips of the probe contactor to be coplanar.) The probe contactor substrate 100 is generally mounted to the probe card assembly 200 in such a manner that the planarity is set by the location of discrete points along the back surface (the surface opposite of the probe contactors) of the probe contactor substrate 100 and thus, due to the non-uniform thickness of the probe contactor substrate, the plane 210 of the probe contactor tips may not be co-planar with the plane 220 of the reference plane as shown in FIG. 2. It is a further problem that due to manufacturing tolerances or errors, the overall heights of the probe contactors can vary linearly across the substrate such that when referenced from the back of a planar substrate, the probe tips lie in a plane that is tilted.
It is a further problem that in applications requiring more than one probe contactor substrate to function in unison, the overall thickness or z distance between the back of each substrate to the tips of the substrates should be identical to eliminate the need to individually compensate the mounting means of the substrates for variation in thickness.
Thus, what is needed is a probe head which has mounting structures (for mounting to the probe card assembly) that are planar to each other and co-planar with the plane of the tips of the probe contactors and are a known predetermined distance from the plane of the tips.